PURPOSE: Normal white blood cell counts (WBC) are unknown in children with acute lymphoblastic leukemia (ALL). Accordingly, 6-mercaptopurine (6MP) and methotrexate (MTX) maintenance therapy is adjusted by a common WBC target of 1.5-3.0 × 10(9)/L. Consequently, the absolute degree of myelosuppression is unknown for the individual child and we wanted to evaluate this.